In golf balls commercially selling, there are solid golf balls and thread wound golf balls. The solid golf ball consists of a core and a cover covering the core. The core may have single-layered structure or multi-layered structure, which has two or more layers. The core is basically formed from rubber material, and can be obtained by vulcanizing and molding rubber composition comprising high-cis polybutadiene, metal salt of α, β-ethylenically unsaturated carboxylic acid (such as zinc acrylate, zinc methacrylate, etc.), peroxide crosslinking agent and the other additives. However, it has been a problem for the resulting molded article to have very poor mold releasability.
In order to improve the mold releasability, many methods of making a golf ball have been suggested in view of the mold and molded article described as follows.
(1) A release agent is sprayed or baked on the surface of a cavity of the mold for molding the core (Japanese Patent Kokai Publication No. 344353/1994).
(2) A release agent is previously dip coated on the surface of a core premolded article (Japanese Patent Kokai Publication No. 344353/1994).
(3) A release agent is previously incorporated in the core premolded article (Japanese Patent Kokai Publication No. 78875/1982).
(4) The mold for molding the core is surface treated with a release agent (Japanese Patent Kokai Publication No. 318491/1993).
(5) The core is molded in the mold through polyester film (Japanese Patent Kokai Publication No. 169487/1999) formed from integrally molded rubber material and a thermoplastic resin cover (e.g. ionomer resin cover) formed on the core.
In Japanese Patent Kokai Publication No. 344353/1994, a release agent for golf a ball comprising
a polyfluorocarbon group containing phosphonic acid represented by the following general formula:RfCnH2nPO(OH)2                 wherein Rf is a polyfluorocarbon group having 3 to 8 carbon atoms and n is 1 to 3, or salt thereof, and        silicone oil or wax dissolved or dispersed in organic solvent or aqueous solvent is disclosed.        
In Japanese Patent Kokai Publication No. 78875/1982, a rubber composition comprising 10 to 60 parts by weight of metal salt of unsaturated carboxylic acid and 0.3 to 0.5 parts by weight of white factice, based on 100 parts by weight of base rubber is disclosed.
In Japanese Patent Kokai Publication No. 318491/1993, a method of making a golf ball comprising the steps of:                providing a molding material for the golf ball in a cavity of a mold for molding the golf ball, which solvent soluble fluoroplastic coating is formed on the surface of the cavity, and        compression molding or injection molding the molding material is disclosed.        
In Japanese Patent Kokai Publication No. 169487/1999, a method of making a golf ball comprising a core and a cover covering the core, the method comprising the steps of:                (a) preparing a core composition,        (b) placing polyester film in a cavity of a mold for molding the core such that the film adapts to the shape of the cavity,        (c) vulcanizing the core composition in the mold through the polyester film to form the core, and        (d) forming the cover on the core is disclosed.        
The method (1) has been generally used, but it has been a problem to have poor productivity, because defective products are caused by the contamination of the release agent in the molded article, or it is needed to clean and remove the release agent which sticks to the molded article and to periodically clean the release agent deposited on the mold, or it has been needed to spray the release agent after every pressing or after pressing several times.
In the methods (2) and (3), it has been a problem that defective products are caused by the contamination of the release agent in the molded article, or it is needed to clean and remove the release agent which sticks to the molded article, as described in the method (1).
In the method (4), there has been, for example, a method by using fluoroplastic coating as a releasing treatment of the surface of the mold, but it has been a problem as follows.                (i) The surface of the mold is roughened by blasting treatment with a blast media, such as alumina, in order to improve the adhesion of the releasing layer to the surface of the mold, but the accuracy of the mold is degraded by repeated blasting treatment every coating step of the releasing layer.        (ii) The durability of the releasing layer itself to the heat and pressure during molding the core is poor, and the releasability can not be maintained for several weeks. Therefore, the manufacturing cost is increased.        (iii) In case of removing the decomposed releasing layer and treating it again as described above, since fluoroplastic is insoluble in solvent and is not removed by the solvent, a physical method such as a blasting treatment is needed, and the removing step costs much time.        (iv) In order to reduce the removing step time, there is a method of putting the mold in a furnace at not less than 400° C. to decompose and carbonize the releasing layer, but corrosive fluorine gas generates, or the mold life is reduced by high temperature treatment.        (v) There is not only the above fluoroplastic coating method, but also a method of applying a solvent soluble fluoroplastic coating (Japanese Patent Kokai Publication No. 318491/1993). However, the durability is poor as compared with expensive material cost, and the manufacturing cost is increased, which affects the profitability.        (vi) Since the baking step is conducted at the temperature of 350 to 400° C., which is not less than the melting point of the fluoroplastic, relaxation of residual stress during processing the cavity occurs, and the cavity may be deformed.        
In the method (5), the continuous use temperature of the polyester resin is low, and it has been problem that the durability of the sheet is poor.